1. Field of the Invention.
The present invention relates to sheet feeding, and more particularly, to a vacuum corrugation feeder with the capability of detecting the degree of curl in sheets.
2. Description of the Prior Art.
With the advent of high speed xerographic copy reproduction machines wherein copies can be produce at a rate in excess of three thousand copies per hour, the need for a document and sheet feeder to, for example, feed documents to the platen of a copier in rapid succession in a reliable and dependable manner in order to utilize the full capabilities of the copier. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the original document and generate minimum machine shutdowns due to misfeeds or document multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur which, in some cases, can be due to upcurl and downcurled in documents.
Since the documents must be handled gently but positively to assure separation without damage through a number of cycles, a number of solutions have been suggested such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such a sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are very positive, the action of the retard member if it acts upon the printed face can cause smearing or partial erasure of the printed material on the document With single sided documents, this does not present a problem as the separator can be designed so that the retard mechanism acts upon the underside of the document. However, with documents printed on both sides, there is no way to avoid the problem. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
Various approaches have been highly successful in answering the above problems, for example U.S. Pat. No. 4,305,576 discloses a typical vacuum separating and feeding system wherein a plurality of friction belts is arranged to run over a vacuum plenum placed at the bottom of a sheet supply tray which has a "U" shaped pocket formed in it. The pocket serves to provide space for the bottom sheet to be captured by the vacuum feed belt assembly, to provide an air seal between the bottom sheet and the edges of the pocket and to provide a high pressure seal between the bottom sheet and the remainder of the stack. This high pressure seal is achieved by supporting a major portion of the stack weight on the edge regions of the pocket. However, this "U" shaped configuration was found to not permit deformation of the sheet in a geometrically developable shape which results in a reduction in the degree of levitation of the sheet stack.
The bottom sheet vacuum corrugation feeder in U.S. Pat. No. 4,411,417 answered this problem by including a differently designed stack support tray that has a planar base portion defining a base plane, the front of the base portion having a opening within which the bottom sheet separator is positioned. The tray also includes two sloping planar side wings, one at each side of the opening in the base portion. The sloping planar side wings are angled upward from the base plane and are angled outward from front to rear of the tray and intersect the base plane such that the intersection at the rear of the tray is in the approximate location of the rear corners of a rectangle the size of a sheet to be fed and the intersection of the planar wings and the base plane at the front of the tray is approximately midway between the front corners of a sheet to be fed and the center line of a sheet to be fed.
The performance of the heretofore mentioned bottom vacuum corrugation feeders (BVCF) depend on the fine balance between the two critical pneumatic parameters, air knife pressure which determines the degree of stack levitation and vacuum pressure which determine the acquisition force and the forward driving force. Any imbalance which favors the air knife will tend to lead to misfeeds while an imbalance favoring the vacuum will lead to multifeeds. In high performance copiers and printers, the air knife pressure is controlled by selecting one of four discrete pressure levels. The input to this controller is the position of the stack height sensor. A high stack will require a high air knife and a low stack will be best served by a low air knife setting. The problem which tends to arise has to do with paper curl. Not only is paper curl more difficult to acquire, since the lead edge may e elevated with respect to the feed belt, but it may also fool the stack height sensor into indicating a high stack. This will give exactly the wrong response, that of a high air knife when the lead edge is already curled away.
U.S. Pat. No. 5,454,556 is one attempt at answering this problem and includes a BCVF with a vacuum switch that changes state at a preset vacuum level thereby detecting any significant rise in vacuum which occurs when a sheet has been acquired from a sheet stack by the BCVF. A digital control circuit senses the change of state which takes place in the vacuum switch and feeds a signal to a machines' microprocessor which in turn signals an air knife in the BVCF to increase or decrease air pressure toward the sheet stack to compensate for the stressed state of the sheets. Even with this improvement, there is still a need for a BVCF that is more sensitive to sheet Curl.